Induction plasma torches have attracted increasing attention as a valuable tool for materials synthesis and processing under high temperature plasma conditions. The basic concept has been known for more than sixty years and has evolved steadily form a laboratory tool to an industrially worthy high power device. Operation of an induction plasma torch involves an electromagnetic coupling of energy into the plasma using an inductive coupling member, for example a 4-6 turns induction coil. A gas distributor head is used to create a proper gaseous flow pattern into the discharge region where plasma is generated. This gaseous flow pattern not only stabilizes the plasma at the center of a plasma confinement tube made, for example of quartz, but also maintains the plasma in the center of the induction coil and protects the plasma confinement tube against damage due to the high heat load from the plasma. At relatively high power levels (above 5-10 kW), additional cooling is required to protect the plasma confinement tube. This is usually achieved using a cooling fluid, for example de-ionized cooling water flowing on the outer surface of the plasma confinement tube.
A standard design of induction plasma torch is illustrated in FIG. 1. the plasma torch of FIG. 1 comprises a cylindrical enclosure surrounded by a water-cooled induction copper coil supplied with a high frequency current. Plasma gas is introduced axially into the inner space of the cylindrical enclosure. As the electrical current flows though the induction coil it creates an axial alternating magnetic field responsible for an electrical breakdown of the plasma gas in the discharge cavity. Once breakdown is achieved a tangential induced current is developed into the plasma gas within the induction coil region. This tangential induced current heats the plasma gas in the discharge cavity to ignite, produce and sustain plasma.
Numerous designs have been developed and experimented to construct induction plasma torches based on essentially the same principles. Various improvements in induction plasma torches are also taught in U.S. Pat. No. 5,200,595 issued on Apr. 6th, 1993 and entitled High Performance Induction Plasma Torch with a Water-Cooled Ceramic Confinement Tube; U.S. patent application Ser. No. 08/693,513 (Aug. 4, 1995) entitled Ignition Device and Method for Igniting a Plasma Discharge in an Induction Plasma Torch; U.S. Pat. No. 5,560,844 issued on Oct. 1st, 1996 and entitled Liquid Film Stabilized Induction Plasma Torch; U.S. Pat. No. 6,693,253 issued on Feb. 17th 2004 and entitled Multi-coil induction plasma torch for solid state power supply; and U.S. Pat. No. 6,919,527 issued on Jul. 19th 2005 and entitled Multi-coil induction plasma torch for solid state power supply, the full subject matters thereof being incorporated herein by reference.
Attempts have also been made to improve the protection of the plasma confinement tube. For example, a segmented metallic wall insert has been used to improve protection of the plasma confinement tube but presents the drawback of substantially reducing the overall energy efficiency of the plasma torch. Also, a plasma confinement tube made of porous ceramic material offers only limited protection. Concerning confinement tubes cooled by radiation, their ceramic materials must withstand relatively high operating temperatures, exhibit an excellent thermal shock resistance and must not absorb the RF (Radio Frequency) magnetic field. Most ceramic materials fail to meet with one or more of these stringent requirements.
A continuing concern with current induction plasma torches is the problem of arcing between the plasma and the exit nozzle of the torch and/or the body of the reactor on which the torch is mounted. A schematic representation of the problem of strike-over is illustrated for both cases in FIG. 2.
More specifically, FIG. 2 illustrates an induction plasma torch including a tubular torch body including a plasma confinement tube for producing plasma. An induction coil is embedded in the tubular torch body. Any powder materials or precursor to be processed in the plasma is injected via a powder injector probe mounted axially through a gas distributor head that sits on top of the plasma torch body. A plasma discharge is produced into a reactor defined by a reactor wall via a water-cooled nozzle. FIG. 2 shows arcing (strike over) between the plasma and the exit nozzle of the torch and the body of the reactor.
An early attempt for solving the problem of arcing in an induction plasma torch was reported by G. Frind in 1991 and was the subject of U.S. Pat. No. 5,233,155 issued on Aug. 3rd 1993. This patent identified that arcing was due to capacitive coupling between the induction coil and the plasma and proposed a solution through the addition of a capacitive shield between the induction coil and the outer surface of the plasma confinement tube. Yet, the introduction of the capacitive shield as proposed by Frind resulted in an increased difficulty of plasma ignition and significant loss of energy coupling efficiency between the coil and the plasma due to energy dissipation in the metallic shield.
There thus remains a need for eliminating arcing without losing energy coupling efficiency and increasing the power/energy density into the plasma discharge cavity.